The invention proceeds from a process for preparing methylenediphenyl diisocyanates (MDI) by reacting aniline with formaldehyde in the presence of hydrochloric acid to give a mixture of diamines and polyamines of the diphenylmethane series and reacting the resulting mixture of diamines and polyamines of the diphenylmethane series with phosgene to give a mixture of diisocyanates and polyisocyanates of the diphenylmethane series.
Aromatic diisocyanates are important and versatile raw materials for polyurethane chemistry. Tolylene diisocyanate (TDI) and MDI are the most important industrial isocyanates here.
The general term “MDI” is used in the technical field and for the purposes of the present invention as collective term for methylene(diphenyl diisocyanates) and polymethylene-polyphenylene polyisocyanates, viz. the polyisocyanates of the diphenylmethane series. The polyisocyanates of the diphenylmethane series are, in particular, compounds and mixtures of compounds of the following type:
where n is a natural number ≧2.
The term methylene(diphenyl diisocyanate) encompasses the isomers 2,2′-methylene(diphenyl diisocyanate) (2,2′-MDI), 2,4′-methylene(diphenyl diisocyanate) (2,4′-MDI) and 4,4′-methylene(diphenyl diisocyanate) (4,4′-MDI). These isomers are referred to collectively as “monomeric MDI”. The term polymethylene-polyphenylene polyisocyanates encompasses “polymeric MDI” (PMDI), which contains monomeric MDI and higher homologues of monomeric MDI.
In all industrially relevant production processes, MDI is obtained by phosgenation of the corresponding polyamines of the diphenylmethane series. The MDI synthesis is usually carried out in a two-stage process. Aniline is firstly condensed with formaldehyde to give a mixture of oligomeric and isomeric methylene(diphenyldiamines) and polymethylenepolyamines, known as crude MDA. This crude MDA is subsequently reacted in a second step with phosgene in a manner known per se to give a mixture of the corresponding oligomeric and isomeric methylene(diphenyl diisocyanates) and polymethylene-polyphenylene polyisocyanates, known as crude MDI. Here, the isomer and oligomer composition remains unchanged. Part of the 2-ring compounds is then usually separated off in a further process step, e.g. by distillation or crystallization, leaving polymeric MDI (PMDI) as residue.
The continuous, batchwise or semicontinuous preparation of polyamines of the diphenylmethane series, hereinafter also referred to as MDA for short, is described in numerous patents and publications (see, for example, H. J. Twitchett, Chem. Soc. Rev. 3(2), 209 (1974), M. V. Moore in: Kirk-Othmer Encycl. Chem. Technol., 3rd. Ed., New York, 2, 338-348 (1978). The preparation of MDA is usually carried out by reacting aniline and formaldehyde in the presence of acid catalysts. Hydrochloric acid is usually used as acid catalyst and the acid catalyst is, according to the prior art, neutralized and thus consumed at the end of the process and before the concluding work-up steps, for example the removal of excess aniline by distillation, by addition of a base, typically aqueous sodium hydroxide. In general, the addition of the neutralizing agent is carried out so that the resulting neutralisation mixture can be separated into an organic phase containing the polyamines of the diphenylmethane series and excess aniline and an aqueous phase containing sodium chloride together with residual organic constituents. The aqueous phase is generally disposed of as inorganically laden wastewater after removal of the organic constituents.
It is known that the composition of the crude MDA prepared by the process outlined, and consequently the crude MDI prepared therefrom, can be varied within a wide range by means of the reaction conditions employed. In this respect, the molar ratio of the components aniline and formaldehyde which are reacted (hereinafter also referred to as A/F ratio for short) and the amount of catalyst added, usually reported as degree of protonation (molar ratio of catalyst added to aniline fed in), are of particular importance.
High A/F ratios make it possible to obtain a crude MDA having a high monomer content, while high degrees of protonation promote the formation of 4,4′-MDA, which is of particular industrial importance as precursor for the corresponding 4,4-MDI. However, both measures lead to formation of large amounts of salts which firstly represents a large economic loss and secondly results in possible pollution of the environment.
There has been no lack of attempts to develop processes in which the wastewater obtained in the preparation of MDA and the salt burden to be disposed of via the wastewater are significantly reduced or avoided entirely.
DE 2 238 920 A1 discloses a process for preparing polyamines of the diphenylmethane series by condensing aniline with formaldehyde in the presence of water and acid catalysts, in which the fully reacted aqueous condensation mixture is extracted with a hydrophobic solvent, the resulting solvent phase is worked up to give the polyamines and the aqueous phase is, after addition of aniline and formaldehyde, recirculated to the beginning of the process. According to the teachings of DE 2 238 920 A1, the water of condensation formed in the reaction of the starting materials and excess water which gets into the system with the starting materials can be discharged from the system at a suitable point. Wastewater which contains essentially only the water introduced with the starting materials and that formed in the reaction of aniline with formaldehyde is obtained in the process; the use of sodium hydroxide is dispensed with. A disadvantage of this process is the use of an additional organic solvent whose use in an extraction and the removal by distillation require a large outlay in terms of apparatus and energy. In addition, large volumes of aqueous phase have to be circulated in the process and likewise have to be worked up by distillation.
The reduction of the aqueous phases to be worked up with simultaneous recirculation of the homogeneous catalyst used is subject matter of DE 2 426 116 A1. According to the teachings of DE 2 426 116 A1, it is particularly advantageous to carry out the reaction of aniline with formaldehyde in a number of stages and to react aniline and formaldehyde in a first stage in the absence of a catalyst to form a precondensate (“aminal”), separate off the water after the reaction is complete, then react the precondensate in the presence of acid salts of aniline or its condensation products with the formaldehyde in the presence of polar, organic solvents which remain inert under the reaction conditions and finally wash the catalyst used from the reaction mixture by means of water and recirculate it to the reaction stage of the precondensate. Here too, disadvantages are the use of an additional organic solvent and the high outlay in terms of apparatus and energy for extraction and distillation required as a result.
The intermediate formation of a precondensate is also disclosed by DE 2 623 681 A1, together with the use of a heterogeneous acid catalyst based on clay, zeolites or diatomaceous earths for the conversion of the precondensate into the polyamines of the diphenylmethane series. According to the teachings of DE 2 623 681 A1, the catalyst can be separated off from the polyamine mixture by filtration or centrifugation after the reaction is complete and recirculated to the reaction stage of the precondensate. However, for the rearrangement reaction, the aminal has to be subjected to complicated predrying, and a further disadvantage of the process is the accumulation of reaction products on the catalyst, which generally results in an unsatisfactory operating life of the catalyst.
Carrying out the conversion of the precondensate of aniline and formaldehyde into a two-ring MDA which is low in polymer using solid acid catalysts and at the same time utilizing the advantages of catalysis using solids having the sufficiently long catalyst operating lives required for industrial processes without obtaining appreciable residual amounts of polymeric MDA bases or incompletely rearranged intermediates (“aminobenzylanilines”) in the end product is likewise subject matter of the teachings of EP 1 257 522 B1. However, disadvantages of the process disclosed in EP 1 257 522 B1 are likewise the costly drying necessary for the precondensate and the accumulation of reaction products on the catalysts used and the resulting short operating lives.
U.S. Pat. No. 7,105,700 B2, too, discloses the use of heterogeneous catalysts in the rearrangement of the aminals which are the primary reaction products of aniline with the formaldehyde added, using surface-modified zeolites as catalysts. However, according to the teachings of U.S. Pat. No. 7,105,700 B2, complicated predrying of the aminals is likewise necessary and the operating life of the catalysts used which is necessary for an industrial scale is achieved by the use of an additional solvent which has to be removed by distillation after the reaction is complete, which is likewise disadvantageous.
The advantages of homogeneous catalysis with simultaneous recovery of the acid catalyst in solid form is disclosed by WO 2005/007613 A1. WO 2005/007613 A1 discloses a process for preparing polyamines of the diphenylmethane series by reacting aniline with an agent which supplies methylene groups, in which the acid used as catalyst is removed from the reaction mixture by means of adsorption, e.g. on an ion exchanger, after the rearrangement reaction is complete and is subsequently recovered by regeneration of the adsorbent. A disadvantage of this process is the high outlay for complete adsorption and desorption of the catalyst used. To achieve complete removal of the catalyst from the reaction mixture, WO 2005/007613 A1 therefore discloses subsequent neutralization with customary basic neutralizing agents in order to remove last traces of acid, so that this known process also requires a high engineering outlay.
In summary, it can be said that in the processes known from the prior art which minimize the salt burden discharged with the wastewater, the outlay for the preparation of MDA becomes greater. In processes using heterogeneous catalysts, the operating life and regeneration of the catalysts generally present great problems, while in processes using homogeneous catalysts, the extraction processes require the use of additional materials and apparatuses. Standard processes using only small amounts of homogeneous catalyst do not give the desired breadth of the isomer composition, and after-treatments which require a high outlay in terms of apparatus and energy are then required.
It is an object of the invention to discover an integrated process for preparing MDI, in which the wastewater obtained in the preparation of the precursor MDA and the salt burden to be disposed of via this wastewater outside the integrated process are significantly reduced or avoided entirely, without the outlay for the preparation of MDI increasing, without the yields, isomer ratios and homologue ratios and their control deteriorating and without the quality of the MDI obtained being adversely affected.